Copy source files to OSX and use gnustep-make

and then compile your project by typing 'make'. You get a native Apple OSX application compiled against the Apple OSX ObjC frameworks ... using
the same GNUmakefiles you were using on GNU/Linux/*BSD/Windows. :-)
--N. Pero

Additionally to the already suggested use of Xcode, you can build GNUstep on OS X with the apple-apple-apple combo.
While this might need a tweak or two to work, this gives you a thin layer of GNUstep (its additions) onto what Cocoa already provides.
--Markus Hitter

Copy source files to OSX to develop, build and debug using Xcode

Add a wrapping Xcode project (in addition to the GNUstep makefile) and configure it to compile directly on OSX for OSX using the OSX Cocoa frameworks.
You can share the sources e.g. through SVN.
There is no problem having GNUmakefiles and some .xcodeproj in the same source code directory.
The main benefit of this approach is that you can use the Xcode/gdb integration for debugging.

Examples using this approach:
1. SWK Browser from the GNUstep SWK project, DataBuilder from
2. GSCoreData (look into the sources at www.gna.org)

Development is done by either working on Linux and using GORM/Project Center and compiling for Linux, or on OSX opening the project in Xcode.
The only thing to keep in mind is that you must also update the Xcode project or the GNUmakefile if you add source files or resources.
The GNUstep .app bundle is different and runs on Linux only.
The OSX .app bundle runs only on OSX, i.e. there is no single bundle that covers all architectures (unless you do some additional tricks).

Install GNUstep on OSX using MacPorts

Then, you can set up an identical build environment on both machines.
The drawback is that you don't have a "native" OSX application which you can easily launch by a double-click on the .app icon.

Benoit Astruc writes: I think I have done this :
- I installed GNUstep Make and BaseAdditions on Mac OS X 10.3 (it was 2 years ago so not the Make 2.0 version, I don't know if it works with
the new version),
- I used a GNUmakefile like this one :

There was a trick however, the first time I had to write manually some file, WRPInfo.plist if I remember well, and then it was used.
I don't do anything on this app since 2 years ago so I don't remember well, but I am pretty sure I had nothing more to do.
And the result was a perfectly Mac OS X app build with GNUstep make.

Real cross-compiling (building on Linux for MacOS X)

This means that you have a gcc version on your Linux machine that emits executables that run on OSX. Unfortunately, OSX uses MACH-O binaries and building a cross-compiling gcc is very tricky.
So I would not consider this as a reasonable option.

I have an Xcode project and want to run on Linux

a) your target machine has a compiler
Here, you have to check that you are not using too specific
frameworks. Then, add a GNUmakefile. Copy the files to your Linux
system and run the GNUstep makefile.

b) your target machine is an embedded system
In this case, you need a cross-compiler on the build host. This can be
a full Linux machine or a OSX machine (but it is more difficult to get
a working cross-compiler running on OSX).